I can't really think of a coaster that currently exists that would beat Leviathan for me. It's too perfect, it's too exactly what I love about roller coasters. I can't imagine ever getting bored of it.
Boulder Dash is a possibility, but Leviathan is way ahead of Voyage, Beast, and Prowler, and those seem to be the coasters it has most in common with (could be totally wrong about that).
I always think it's so hard to compare roller coasters, since the variety is completely unmanageable and you can't ride more than one at a time to get a fair comparison, and they all ride differently every day of the week anyway. It's a bit easier to think in terms of genres, like speed vs. intensity, low-to-the-ground vs. big hills, airtime vs. positives, etc. I tend to favor speed, big hills, and airtime, so naturally Leviathan is a great fit for me.